The present invention relates to a nuclear reactor used for power generation, research and the like, and in particular, a liquid-metal cooled nuclear reactor using liquid metal coolant such as sodium wherein the integrity or soundness of its coolant pressure boundary is improved and maintained.
A fast breeder reactor using liquid sodium as a coolant will be explained as a typical example of liquid-metal cooled reactors. FIG. 2 is a schematic diagram illustrating the fast breeder reactor "Monju", wherein heat generated in a reactor vessel 1 including a reactor core 2 therein is transferred to a primary sodium coolant contained in the reactor vessel 1. Heat is then transferred to a secondary sodium coolant via an intermediate heat exchanger 3, and further to water in an evaporator 4 to generate steam therein. The steam is supplied, via a superheater 5, to a turbine 6 to drive a generator 7, thus generating electricity. The steam coming out of the turbine 6 is condensed into water in a condenser 8.
The primary sodium system (encircled by single-dot and dash lines in FIG. 2) is composed of a primary sodium system pump 9 and a primary sodium system main piping 10 which in combination circulate primary sodium coolant through the reactor vessel 1 and the intermediate heat exchanger 3.
A secondary sodium system is composed of a secondary sodium system pump 11 and a secondary sodium system main piping 12 which in combination circulate secondary sodium coolant through the intermediate heat exchanger 3, the evaporator 4, and the superheater 5. Further, a water/steam system is composed of a water feed pump 13 and a water/steam main piping 14 which in combination circulate the water/steam through the evaporator 4, the superheater 5, the turbine 6 and the condenser 8. The primary sodium system is usually installed in a nuclear reactor containment vessel, whereas the secondary sodium system and the water/steam system are installed outside the reactor containment facility.
The coolant pressure boundary, i.e. the boundary between the vessels and piping containing the primary or secondary sodium coolant and the external region thereof, is extremely important in terms of safety. Therefore, in the design and manufacture of such a nuclear reactor as described above, it is imperative to be very careful not to allow the sodium to leak from the vessels and the piping, and the materials of these vessels and pipings should be of steel having a great strength at high temperature and having excellent resistances to sodium corrosion, neutron irradiation, thermal shock and the like.
Further, in order to better deal with an emergency that would occur should there be leakage across the coolant pressure boundary, there have been provided with various precautionary measures such as the installation of the main piping 10 at higher locations, the arrangement of guard vessels 15 around the reactor vessel 1, the intermediate heat exchanger 3 and the primary sodium system pump 9, and further substitution of nitrogen for the ambient air. These countermeasures, however, more or less compromise the economical efficiency of the sodium-cooled reactor.